C64 common problems and how to fix it

On this page you can find all the gathered information I found on the web to fix your Commodore 64. There are pictures with the common problems, but also descriptions of failing parts of the C64. Hope this helps to fix your Commodore.

To the original author(s):
I’m just preserving your valuable information. Just in case it would ever go offline.
As always I would like link and point out to the ORIGINAL sources:

Over time I’m collecting more tips and screenshots from various sites and videos. Good luck finding the solutions you need.

C64 Board reference to all U-chips

Non-fault #1 – Red/Green Bars

Description
Whenever grey raster (or anything with rapidly alternating pixels) is drawn, the grey color instead appears as bars or red and green. This is not due to a fault in the C64 itself. It happens when composite video or RF is used and the high frequency pattern is misrepresented as color information. Using S-Video should fix or lessen this problem, as it keeps the brightness and color components separate.

Cause
Color bleeding of Composite/RF

Non-fault #2 – Graphical errors in Games

Description
In some parts of Giana Sisters, graphical errors blink for a fraction of a second. This is just a bug in the software, and can even be reproduced with emulators. In fact, 99% of the time when you see simple graphics anomalities like this in games, it is simply caused by unpolished software. So before suspecting your C64 hardware, make sure it’s not actually normal behavior for the game/program in question. NOTE: the video is slowed down to make it easier to see the errors.

Cause
Unpolished software

Non-fault #3 – Video out of sync

Description
Computer operated normally, except video output was glitched, as if it were out of sync. Both RF and Composite displayed the same problem. Swapping in another VIC chip and re-capping did not fix it either. It turned out to be a compatibility problem with the LCD TV. The television worked fine in other use, but would not work properly with a C64. NOTE: ‘Device not present error’ not related to this problem.

Non-fault #4 – fuzzy colors

Description
Fuzzy colors on video output. This happens if you connect a C64 to a TV with an S-Video cable, but the TV either doesn’t support it, or has it switched off. The fuzzy colors are caused by luminance/chrominance leakage.

Cause
Using S-video cable with a TV that doesn’t support S-video, or S-video mode not switched on

Also; it’s best to use an original Commodore 1701 or 1702 monitor. It has Luma and Chroma inputs. it would be best to use a L/C/A cable.

CIA1 Faults U1

Descriptionimage 1:
Relatively normal startup screen, but frozen with a rapidly flashing cursor and ‘LOAD’ command run automatically. No response to keyboard commands. A cartridge game loaded up properly but joystick and keyboard did not work. Picture re-created with VICE C64 Emulator.

Image(s) by Ozsoft @ lemon64.com

images 2,3,4:
Caused various abnormal startup screens, including a simple black screen on a couple of boots. The most common outcome was an empty blue screen with borders (first picture). After about 10 seconds of waiting, most of the normal startup text appeared along with some garbage characters, and in some cases automatically typed load/run commands (second picture). Sometimes the characters kept switching between upper and lower case. On one boot, screen was filled with white garbage characters (third picture). The computer seemed unresponsive on every boot.

Though in this case the problems were simply caused by a bad connection between the CIA1 chip and its socket, similar symptoms can likely be caused by a failing CIA chip.

General
Startup screen normal, but no cursor. No keyboard or control port access but cartridge works. Partial failure: some keys or joystick positions don’t work, one character appears ahead of startup cursor or random characters appear at startup. Blank screen if chip is shorted (remove to check) and chip may get hot to the touch.

CIA2 Faults U2

Description
Normal borders, but garbage in the middle. Sometimes after a while or when you try to load something from disk (the drive won’t start to load).

General
Startup screen normal. No serial or user port access. “File not found” error when drive accessed. Cartridge works. Characters may show as blocks on startup screen.

Cause
– Bad U2 906108-02 (6526) CIA

Basic ROM Fault U3

Description
Startup screen with normal blue colors and borders, but content area completely empty. A cartridge game worked normally. It turned out to be a faulty Basic ROM chip. A blank blue screen like this is the most common symptom of a bad Basic ROM. Cartridges bypass the Basic ROM so they are good for diagnosing problems with that chip.

Image(s) by danko @ lemon64.com

General
Blank screen w/ border. Cartridge works.

Cause
– Bad U3 (Basic ROM) 901226-01

Basic KERNAL Fault U4

Description

Normally booted to a black screen. With Jupiter Lander game cartridge inserted, produced the garbage screen shown above. Removing the Kernal ROM allowed Jupiter Lander to boot normally, and replacing that chip resolved the issue. Note that Jupiter Lander bypasses the Kernal ROM and is usually not affected by a fault in that chip. But as this fault proves, it’s still possible.

General
Blank screen with border or screen full of shimmering lines or characters. Partial failure: “garbage” characters or blocks where startup page info should be. Cartridge works.

Cause
Bad U5 (Character ROM) 901225-01

Color RAM fault U6

DescriptionStartup screen with some missing characters. Certain columns are missing. Games also had missing characters. The computer was mostly functional, commands worked normally. Diagnostic Cart Rev 586220 froze when it reached the Color RAM test. Swapping in a new Color RAM chip fixed the problems. Image(s) by jts-78 @ com64.net

other screenshot
Startup screen was normal and the computer was operational, but changing the cursor color to white revealed a problem: another character also turned white. Which character was drawn white depended on the position of the cursor. In addition, games had abnormal graphics. Replacing the Color RAM fixed the symptoms. Image(s) by lolhead @ lemon64.com

3rd screenshot
System worked fine except that colors were incorrectly displayed. Problem went away after U6 color ram was replaced. Image(s) by zxspectrum_16k @ lemon64.com

Logic fault U8

2nd:
Machine was working normally, except that it hung when trying to load via a disk drive. There was no reaction from the drive. The screenshot depicts the point where it hung. Swapping the CIA chips U1 and U2 did not make a difference. Replacing the 7406 logic chip at U8 finally resolved the problem.

RAM Problems U9 – U12, U21 – U24

Description
Startup screen problems with:

out of memory error,

wrong number of memory available,

colored random characters.

General
Blank screen, no border. Shorted chips may get warmer (sometimes very hot) than the other RAM chips. Partial failure: will sometimes produce “garbage” screen, abnormal number of bytes free (lower than 38911) or “OUT OF MEMORY IN 0” error on startup screen.

Logic problem – U13

Description
A startup screen full of random characters or strange images, similar to one caused by bad RAM. In most cases the C64 still works. Also in many cases the C64 works with a (deadtest) cartridge.

Cause

Bad Logic Chip U13

Logic problem – U14

Description
A startup screen with incorrect characters, you’ll see most of the time that there’s incorrect placement of text. In all cases the Commodore does function correct, but incorrect display of characters.

PLA problem – U17

SID problems – U18

Description
A defective SID can (next to no sound or digi) produce a black screen. You can debug to remove the SID and turn the C64 on.

General
Blank or “garbage” screen if shorted (remove to test), otherwise normal screen. No sound or garbled sound. Mouse or graphics tablet pointer stuck or jitters. NOTE: for test purposes, computer will work without a SID plugged in. NOTE: 8580 and 6581 are pin compatible and somewhat interchangeable, but only with board component changes: pin 28 supply voltage either +9 or +12VDC, and two capacitors values must be changed for correct voicing. No sound: also check capacitor C77 (open). Never ever interchange a 6581 with a 8580 (without proper modifications), it can and will kill your C64!

Cause

Bad SID Chip U18

VIC problems – U19

Logic problem – U25

Description
Initially had a black screen on startup, which was fixed by a PLA swap. After that, random black/blue characters (shown above) displayed on startup. Dead test cartridge worked and the memory test completed successfully. Swapping in new RAM, U13 and PLA did not cure this symptom. Replacing U25 finally did the trick and the machine booted up normally. The failed U25 was a MOS version of the chip, which are known to fail more often than their non-MOS counterparts.

Cause

Bad Logic Chip U25

Logic problem – U26

Description
Seeing distortion smaller than the size of characters, then it could be the Logic Chip U26

Cause

Bad Logic Chip U26

Oscillator problem – U31

MOS 8107 Clock generator

Description
Black screen? If you turn on the c64 and you don’t see that common flash before the black screen. Chances are that the MOS 8701 Clock generator is faulty. Or blank white screen and no border.

Cause

8701 or 7701 MASTER CLOCK OSC.

Source
Thanks to MindFlareRetro

4066 CMOS QUAD SWITCH

Description
Wrong colors on characters.

Cause

4066 CMOS QUAD SWITCH

Source
http://personalpages.tds.net/~rcarlsen/cbm/c64/c64c-ic.txt

Tips & info

Piggy back an IC

The piggy back method gives you an educated guess identifying the faulty IC. Just push a known good memory chip over the chip to be tested. If the fault changes/improves, you have a good chance of pin pointing the faulty chip without soldering. This method has been done many times in the Arcade scene / repairs.

note: do this at your own risk btw 🙂

Black screen?

Do you see a flash? No, replace the clock generator.

If you see a flash on power-on, first try to remove the SID; still doesn’t work?

Replace the VIC chip; still doesn’t work?

Replace the PLA (906114-01); still doesn’t work?

check this video:

another problem can be your power supply, here’s how you can test your power supply:

Cassette problems (datarecorder)

Power supply (PSU) issues

POWER PACK: +5VDC at 1.5Amps and 9VAC at 1 Amp
Can produce many problems like blank screen (red power LED on, dim or off), program lock-up, “garbage” screen, hum bars moving on screen, hum in audio, damaged RAM chips, intermittant operation after warmup, etc. As common a failure as it is, the supply should be checked (by substitution) first.

Pict-o-guide

Commodore Diagnostician II

This information was present on Commodore.ca, but it’s gone (at least at the link I had), therefore I preserved this on my site from the archive.org. All credits of course go to the original authors (see yellow cell below)